Corrosion resistant mercury-free zinc anode battery

ABSTRACT

An electrochemical cell comprising a zinc electrode in an electrolyte solution wherein the electrode is formed from a zinc powder compacted to a density of at least about 6.5 g/cc to substantially reduce the corrosion of the zinc electrode and the consequent evolution of hydrogen gas without resorting to the addition of mercury to the electrode.

This is a divisional of co-pending application Ser. No. 839,716 filed onMar. 14, 1986, now U.S. Pat. No. 4,649,093.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates to electrochemical cells and moreparticularly to a corrosion resistant battery with a zinc powder anodeand an alkaline or an acidic electrolyte, and to a method for itsmanufacture. Applicants' novel battery is corrosion-resistant andcombines the cost benefits and efficiency of a zinc anode while avoidingthe hazards normally associated with adding mercury to the zinc in orderto suppress corrosion. The battery is manufactured by compacting zincpowder to a density of about at least 6.5 g/cc.

B. Description of the Prior Art

Zinc was the first and also the most common material used as a negativeelectrode when converting electrochemical energy in both non-rechargableand rechargable batteries. Zinc metal offers a very attractive range ofproperties for battery application, including low cost, no toxicity,ease of fabrication, high energy density, low electronegativity (highcell voltage) and high exchange current density.

When zinc metal is in contact with an aqueous alkali, anodic dissolutionof zinc metal and cathodic evolution of hydrogen gas occursimultaneously. The former reaction causes zinc oxide or zinc hydroxideto form. The oxides then react with excess hydroxide in the bulkelectrolyte to form a soluble complex anion called zincate (Zn(OH)₄)⁻² :

    Zn+4OH.sup.- →(Zn(OH).sub.4).sup.-2 +.sup.2e-       ( 1)

At the same time, water is reduced to hydrogen gas:

    2H.sub.2 O+2e-→H.sub.2 +2OH.sup.-                   ( 2)

This combination of reactions institutes the corrosion reaction with theevolution of hydrogen gas:

    Zn+2OH.sup.- +2H.sub.2 O→Zn(OH).sub.4.sup.-2 +H.sub.2 ( 3)

The self discharge reactions described above are detrimental, not onlybecause they reduce the dischargeable capacity (energy) of the batterywith time, but also because they require incorporation of a hydrogen gasventing system. This, in turn, makes the battery more prone todeterioration by means of evaporation of the electrolyte.

It has been a common practice for many years in zinc battery technologyto add metallic mercury to the zinc electrode to suppress the evolutionof hydrogen gas. Mercury-zinc alloy (zinc amalgam) has a lower corrosionrate than pure zinc. Mercury, however, is both highly toxic andvolatile. Unusual safety precautions are necessary during themanufacture of products containing mercury. An additional problemassociated with the use of mercury is the disposal of productscontaining that toxic substance. Consequently, complex disposaltechniques must be employed for products containing more than 0.2 ppm ofmercury to reduce the risk of ground water contamination.

The elimination of mercury from commercial batteries utilizing zinc asthe anode is, therefore, considered to be a desirable objective. Theadvantages that result from the elimination of mercury include costsavings associated with the processing of zinc anodes and the disposalof the spent battery materials.

There have been other attempts in the art to mitigate the hazards andadded expense associated with the evolution of hydrogen gas inelectrochemical cells with zinc electrodes. Those attempts have focusedupon the use of additives, either to the alkaline electrolyte solutionor to the zinc anode itself. For instance, U.S. Pat. No. 4,377,625 toParsen et al discloses the addition of aminocarboxylic acid, polyamineor aminoalcohol chelating agents which are said to combine by acoordinate bond with the zinc in the electrolyte solution. However, itis unclear how such bonding would inhibit corrosion of the anode and theconsequent evolution of hydrogen gas.

U.S. Pat. No. 3,580,740 to James discloses a pressed powder zincelectrode with the addition of from about 1 to 10% by weight leadsulfide to reduce hydrogen gassing at the zinc electrode.

James' experiments reveal, however, that the addition of lead sulfide tothe zinc anode was not as effective in reducing hydrogen gas evolutionas is the addition of mercuric oxide. Indeed, the use of a mercuryadditive in the anode is the most common approach in the art for thepurpose of inhibiting corrosion and hydrogen gas evolution. See McBreen,Electrochimi., Icta, 26: 1439-1446 (1981); U.S. Pat. No. 3,870,564; U.S.Pat. No. 4,339,512. Several research projects have concluded thatmercury is the only effective additive in increasing electrode dischargecapacity under all discharge conditions, e.g., Dirkse and Shoemaker, J.Electrochemical Society, p. 115 (August, 1968); Shepard, "Silver ZnAlkaline Prim. Cell", part IV, NRC Report 4885, Naval Research Lab,Washington, D.C. (February, 1957). As stated above, a mercury additiveto anodes is extremely toxic and volatile. Thus, unusual safetyprecautions must be taken at both the manufacture and disposal stages.

SUMMARY AND OBJECTS OF THE INVENTION

In view of the foregoing limitations and shortcomings of the prior artcompositions, as well as other disadvantages not specifically mentionedabove, it should be apparent that there still exists a need in the artfor a battery which combines the advantages of a zinc anode without theharmful effects of mercury additives.

It is therefore a primary object of the present invention to provide abattery with a zinc anode, without resorting to the addition of mercury.

An additional object of the present invention is to provide a zinc anodebattery in which the anode is resistant to the corrosive effects of anacidic electrolyte solution.

Another object of the present invention is to provide a zinc anodebattery in which the anode is resistant to the corrosive effects of analkaline electrolyte solution.

Yet another object of the present invention is to provide a battery inwhich there is no need for a system to vent hydrogen gas.

Still another object of the present invention is to provide aninexpensive, non-toxic battery.

And yet another object of the present invention is to provide a batteryin which the anode is easily fabricated.

And still another object of the present invention is to provide abattery in which the anode has a high energy density, a lowelectronegativity and a high exchange-current density.

And yet another object of the present invention is to provide a batterywith a zinc anode of fine grain size.

A further object of the present invention is to provide a method bywhich the batteries described above can be manufactured.

Briefly stated, the present invention comprises a current-producing cellin which the anode comprises a zinc powder which has been compacted to adensity of about at least 6.5 g/cc. Such density is achieved bycompacting zinc powder at pressures of at least 33,000 psi. Bestresults, however, are achieved by compacting zinc powder at pressures ofat least 65,000 psi to achieve a density of 6.8 g/cc. Increasingpressures above the level of 130,000 psi does not significantly improvezinc density. It has been further found in accordance with thisinvention that density is also affected by other factors in addition topressure, including: zinc particle geometry, temperature of the zinc attime of compaction and particle size distribution. Batteries of thepresent invention in which the zinc powder has been compacted to adensity of 6.5 g/cc show an initial rate of hydrogen gas evolution of0.011 milliliters/cm² /hour or less. At densities of 6.8 g/cc, the rateof hydrogen gas evolution drops to 0.009 milliliters/cm² /hour.

The batteries of the present invention have several advantages overthose of the prior art. Most clearly, the use of compacted, atomizedzinc powder anodes minimizes loss of performance through electrodeerosion without incurring the safety problems associated with the use ofmercury. Moreover, production is streamlined. There is no need forfoundry or machining steps. Further, the pressing process isuncomplicated and can be automated to reduce labor costs. Finally,compaction involves almost no waste of zinc, in contrast to alternativecasting methods wherein scrap often exceeds 50%.

DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a single plunger press.

FIG. 2 is a cross-sectional view of a double plunger press.

FIG. 3 is a graph depicting the density of compacted zinc powder as afunction of applied compacting pressure.

FIG. 4 is a graph depicting hydrogen gas evolution of various anodes asa function of time.

FIG. 5 is a graph depicting initial hydrogen gas evolution rate ofvarious anodes as a function of applied compacting pressure.

FIG. 6 is a graph showing the initial hydrogen gas evolution rate as afunction of density of compacted zinc powder.

FIG. 7 is a graph showing hydrogen gas evolution for zinc powder ofvarious particles size distributions as a function of time.

FIG. 8 is a graph showing the weight loss due to corrosion as apercentage of total weight loss as a function of applied compactingpressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is directed to a current producing cell in whichthe anode comprises zinc powder that has been compacted to a density ofat least 6.5 g/cc. The cell includes a container which serves as ahousing. Connected to the container is a positive terminal electricallyassociated with a cathode inside the container. Also connected to thecontainer is a negative terminal which is electrically associated with azinc anode inside the container. Substantially surrounding andconnecting the cathode and anode is an alkaline or an acidic electrolytesolution.

There are substantial improvements in the corrosion resistance of zincanodes which comprise zinc powder which has been compacted to a densityof at least about 6.5 g/cc.

In traditional casting methods, molten zinc is cooled nonuniformly. Theouter areas in contact with the mold surface cool quickly and thereforesolidify with a relatively small grain structure. Inner areas, however,cool more slowly and consequently solidify with a much larger grainstructure. The rate of cooling also affects impurity distribution.Faster cooling affords impurities less time to collect and thereforeresults in a widely-dispersed and uniform impurity distribution. Inareas of slower cooling, impurities tend to migrate and collect at thegrain boundaries with a consequent nonuniform distribution due tolocalized cluster formation. It is believed that zinc corrosion andhydrogen gas evolution are related to zinc grain size and impuritydistribution.

Large grain size results in deeper crevices between zinc grains. Whenzinc reacts with the electrolyte solution toward the lower ends of suchcrevices, the electrolyte is depleted in those regions. As a result ofthe depths of the crevices, the electrolyte is depleted in those areas,since outside electrolyte cannot freely circulate deep into the crevicesto replenish the lost electrolyte. This electrolyte depletion causes thezinc corrosion reaction to accelerate. (See Equation 2).

With small grain zinc, the shallow crevices permit faster electrolytecirculation to preclude electrolyte depletion. Consequently, there ismuch less acceleration of the corrosion reaction.

The data graphically depicted in FIG. 6 demonstrate a sharp drop in theinitial rate of hydrogen gas evolution for zinc powder compacted to adensity of about 6.5 g/cc. There are further marginal decreases inhydrogen gas evolution as the density of the compacted zinc powderapproaches its theoretical limit of approximately 7.12 g/cc. Althoughseveral factors affect the density of compacted zinc powder, the singlemost important factor is the degree of compaction pressure applied. Asdisclosed by the data in FIG. 3, applied pressure of about 33,000 psicompacts zinc powder to a density within the range of the presentinvention. Density increases at a diminishing rate with increasingapplied pressure, and very little increase in density is noted withapplied pressures in excess of 130,000 psi. The significance ofcompaction at pressures in excess of 33,000 psi is demonstrated in FIG.4 in which hydrogen gas evolution over time is plotted for samples ofzinc powder compacted at different pressures. Those samples compacted atconventional pressures as well as the sample cast with mercury exhibitsubstantially higher levels of hydrogen gas evolution than those samplescompacted at pressures of 65,000 psi and greater. Moreover, zinc powdercompacted at 33,000 psi exhibits levels of hydrogen gas evolution whichare substantially the same as that for cast zinc amalgamated withmercury, without the problems associated with mercury. Similarly, thedate depicted in FIG. 5 shows that for various samples of zinc powdercompacted at different pressures as in FIG. 4, there is a substantialdecrease in the initial rate of hydrogen gas evolution at densities of6.5 g/cc and greater.

It has been determined in accordance with this invention that anotherfactor affecting density, and therefore hydrogen gas evolution, is theinitial particle size distribution and geometry of the zinc powder. Fora given pressure, the wider the distribution of particle size, the lowerthe level of hydrogen gas evolution. When particles cover a wide rangeof particle size, they achieve a greater density since the smallerparticles tend to fall within the interstices of the larger particles.The data depicted in FIG. 8 show that zinc powder covering a widedistribution of particle size evolves far less hydrogen gas than dothose with a more narrow distribution range. Also producing good resultsis zinc powder in a flake shape, because of the relative ease with whichit may be compacted and the consequently greater density to which it canbe compacted at a given temperature.

Although not essential to the practice of the present invention, it isdesirable to compact the zinc powder using a steel die with two plungersrather than just one plunger. The double-action effect of a two plungersystem increases the density of the zinc powder uniformly. Suchuniformity is believed to reduce the rate at which hydrogen gas isevolved. Uniformity of density is also a function of the length andwidth of the compaction chambers. The lower the ratio of length towidth, the more uniform the density distribution of the resulting zincpowder. This ratio should be 2, preferably 3/2, more preferably 1 andstill more preferably 1/2. Most preferable is the application ofisostatic pressure through a compaction device currently available fromUnion Carbide. For mass production of anodes a roller system of plungersmay be most desirable. Additionally, density may be increased if thezinc powder to be compacted is first heated to a temperature higher thanroom temperature, but lower than melting temperature.

The practice of this invention may be more clearly understood from thefollowing nonlimiting illustrative examples.

EXAMPLE 1 [SHOWN IN FIG. 3]

Six samples of zinc powder of a size distribution in the range of-24+200 (U.S. Standard Scale) were placed in a die in a mold andcompressed at room temperature under different pressures for a period of60 seconds. The density of each sample was then measured by a waterdisplacement method. The densities of the samples as a function ofpressure applied are shown in FIG. 3.

EXAMPLE 2 [SHOWN IN FIG. 4]

Samples of zinc powder of a size distribution in the range of -24+200were placed in a mold and compressed at room temperature at variouspressures. Also prepared were an uncompacted zinc cast samplesamalgamated with 0.6% mercury. Each of these samples was tested forhydrogen evolution over time. The results of the tests are shown in FIG.4 and reveal a substantial reduction in hydrogen evolution for zincpowder compressed at pressures of 65,000 psi or higher. Although zincpowder compressed at a pressure of 33,000 psi does not show betterhydrogen gas evolution properties than cast zinc amalgamated with zinc,it does not suffer problems associated with mercury batteries. Thelowest rate of hydrogen gas evolution is for uncompacted zinc powder.This result is consistent with our theory of electrolyte depletion sinceelectrolyte circulates freely through uncompacted powder and there islittle risk of electrolyte depletion. Of course, uncompacted zinc powderis unformed and would not be a practical material for an anode.

EXAMPLE 3 [SHOWN IN FIG. 5]

Samples of zinc powder were compacted as in Example 2 and tested forthis initial rate of hydrogen evolution. The data, which are shown inFIG. 5, demonstrate substantially lower rates of hydrogen gas evolutionwith compacting pressures above 65,000 psi. The rate of hydrogen gasevolution for zinc powder compacted at 33,000 psi is approximately thesame as that for cast zinc amalgamated with mercury.

EXAMPLE 4 [SHOWN IN FIG. 6]

Samples of zinc powder of various densities were tested for theirinitial rate of hydrogen gas evolution. Those rates, as a function ofdensity, are show in FIG. 6.

EXAMPLE 5 [SHOWN IN FIG. 7]

Samples of zinc powder of various particle size distributions werecompacted at room temperature at 65,000 psi for 30 seconds and at 98,000psi for 30 seconds. These samples were then tested for hydrogen gasevolution over time. The results are shown in FIG. 7 and support ourtheory that electrolyte depletion affects hydrogen gas evolution. Zincpowder in a relatively narrow particle size distribution range is not aseasily compressed at a given pressure as is zinc powder in a relativelywide particle size distribution range. When zinc particles are of a widevariety of sizes, the smaller particles tend to compact within theinterstices of the larger particles, thereby increasing density andreducing the depth of the crevices between the particles. As would beanticipated by our theory, zinc powder with a size distribution -230 (anarrow range) showed greater hydrogen gas evolution than did zinc powderwith a -135 size distribution (a wider range). Similarly, zinc powderwith a -135+200 size distribution (a narrow range) showed greaterhydrogen gas evolution than did zinc powder with a - 135+325 sizedistribution (a wider range). The flat geometry of zinc flakes (+24 sizedistribution) allows them to pack together tightly and producerelatively little hydrogen gas despite a narrow particle sizedistribution. Nevertheless, the least hydrogen gas evolution occurs withzinc powder in the widest particle size distribution tested (-24+200).

EXAMPLE 6: [WEIGHT LOSS COMPARISON]

                  TABLE I                                                         ______________________________________                                        Comparison of Compacted Zinc Powder Anode                                     and Casted Zinc Anode Systems                                                 Continuous Discharge at 0.7 A (5.5 mA/cm.sup.2) and RT                                                Weight                                                       Zinc Anode                                                                             Total   Loss      Weight Loss                                        Operating                                                                              Weight  Due To    Due To                                             Potential                                                                              Loss    Discharge Corrosion                                          (volt vs SCL)                                                                          (gm)    (gm)      (gm)  (%)                                   ______________________________________                                        Zinc Cast                                                                              -1.533     1207    823     384   31.8                                +0.6% Hg -1.538     910     837     73    8.0                                 Compacted                                                                     Zinc Powder                                                                   Anode                                                                         12,500 psi                                                                             -1.535     627     528     99    15.8                                21,000 psi                                                                             -1.535     918     837     81    8.8                                 32,000 psi                                                                             -1.536     848     818     30    3.5                                 48,000 psi                                                                             -1.536     830     818     16    1.9                                 64,000 psi                                                                             -1.537     830     818     12    1.5                                 ______________________________________                                    

These measurements support the results obtained by hydrogen gasevolution measurements in Examples 3 and 4 above. A significantreduction in weight loss due to corrosion occurs with anodes compactedat about 32,000 psi and above.

Based on the foregoing tests, it is clear that the inventive batteriesdescribed herein combine the positive qualities of zinc anodes withoutthe problems typically associated with the addition of mercury or theevolution of hydrogen gas.

Although a preferred form of the present invention has been illustratedand described, it should be understood that the invention is capable ofmodification by one skilled in the art without departing from theprinciples of the invention. Accordingly, the scope of the invention isto be limited only by the claims appended hereto.

What we claim is:
 1. A method for the manufacture of a zinc anodebattery in which corrosion of the zinc electrode and the evolution ofhydrogen gas therefrom during discharge of the battery are substantiallysuppressed, comprising the steps of providing a given volume of zincpowder, said volume having a density less than about 6.5 g/cc,compacting the given volume of zinc powder into an anode for a zincanode battery with a volume smaller than the given volume, said anodehaving a density of about 6.5 g/cc or greater, and making a zinc anodebattery using said compacted zinc anode.
 2. The method of claim 1wherein the zinc powder is compacted to a density of about 6.8 g/cc orgreater.
 3. The method of claim 1 wherein the zinc powder has beencompacted at a pressure of about 33,000 psi or higher.
 4. The method ofclaim 1 wherein the zinc powder has been compacted at a pressure ofabout 65,000 psi or higher.
 5. The method of claim 1 wherein the zincpowder has been compacted at a pressure of about 100,000 psi or higher.6. The method of claim 1 wherein the zinc powder has been compacted at apressure of about 130,000 psi or higher.
 7. The method of claim 1wherein the zinc powder has a particle size distribution ofapproximately -24+200 on the U.S. Standard Scale, Procedure ASTM-E11before compaction.
 8. The method of claim 1 wherein the zinc powder isin the shape of flakes before compaction.
 9. The method of claim 2wherein the zinc powder has a particle size distribution ofapproximately -24+200 on the U.S. Standard Scale, Procedure ASTM-E11before compaction.
 10. The method of claim 2 wherein the zinc powder isin the shape of flakes before compaction.
 11. The method of claim 1wherein the zinc powder is compacted in a press with a multiplicity ofpistons.
 12. The method of claim 11 wherein the press has a length towidth ratio of 2 or less.
 13. The method of claim 11 wherein the presshas a length to width ratio of 3/2 or less.
 14. The method of claim 11wherein the press has a length to width ratio of 1 or less.
 15. Themethod of claim 11 wherein the press has a length to width ratio of 1/2or less.
 16. The method of claim 1 wherein the zinc powder has beenheated to a temperature above room temperature but below its meltingpoint at the time of compaction.
 17. The method of claim 2 wherein thezinc powder has been heated to a temperature above room temperature butbelow its melting point at the time of compaction.
 18. The method ofclaim 3 wherein the pressure of compaction is substantially isostatic.19. The method of claim 4 wherein the pressure of compaction issubstantially isostatic.
 20. The method of claim 1 wherein the steps ofcompacting is achieved by a roller system.
 21. The method of claim 2wherein the step of compacting is achieved by a roller system.
 22. Amethod for substantially suppressing corrosion of a zinc electrode of azinc anode battery and evolution of hydrogen gas from the zinc electrodeduring discharge comprising the steps of providing a given volume ofzinc powder, said volume having a density less than about 6.5 g/cc,compacting the given volume of zinc powder into a volume smaller thanthe given volume, said smaller volume having a density of about 6.5 g/ccor greater, forming a battery electrode from said compacted volume ofzinc powder and using said compacted zinc battery electrode to make azinc anode battery.
 23. A new use of compacted zinc powder as anelectrode in a zinc anode battery to substantially suppress corrosion ofthe zinc anode and the evolution of hydrogen gas therefrom duringdischarge of the battery.